Method and system for vacuum control

Abstract

Methods and systems are provided for controlling and coordinating control of a post-catalyst exhaust throttle and an EGR valve to expedite catalyst heating. By closing both valves during an engine cold start, an elevated exhaust backpressure and increased heat rejection at an EGR cooler can be synergistically used to warm each of an engine and an exhaust catalyst. The valves may also be controlled to vary an amount of exhaust flowing through an exhaust venturi so as to meet engine vacuum needs while providing a desired amount of engine EGR.

The invention claimed is: 
     
       1. A method for an engine, comprising:
 during an engine cold-start, closing a post-catalyst exhaust throttle and an EGR valve while diverting at least a portion of throttled exhaust gas through an EGR cooler, the EGR cooler coupled to an exhaust passage upstream of the exhaust throttle, the EGR valve coupled to an EGR passage downstream of a junction, the diverted portion directed to a bypass passage; and 
 during engine operation, coordinating closing of the exhaust throttle and the EGR valve to maintain an amount of EGR delivered to the engine, wherein the cold-start closing of the exhaust throttle and EGR valve and diverting through the EGR cooler is performed for a duration until a temperature of an exhaust catalyst is above a threshold temperature. 
 
     
     
       2. The method of  claim 1 , wherein the coordinated closing is based on an engine vacuum need. 
     
     
       3. The method of  claim 1 , wherein the coordinated closing of the exhaust throttle includes fully closing the exhaust throttle. 
     
     
       4. The method of  claim 1 , wherein the exhaust gas diverted through the bypass passage is not recirculated to an engine intake. 
     
     
       5. The method of  claim 4 , further comprising, while the temperature of the exhaust catalyst is below the threshold temperature, intermittently opening the exhaust throttle in response to an exhaust back-pressure estimated upstream of the throttle being higher than a threshold pressure. 
     
     
       6. The method of  claim 4 , wherein the EGR passage is a low pressure EGR passage. 
     
     
       7. The method of  claim 4 , wherein the bypass passage is coupled to the EGR passage upstream of an outlet of the EGR cooler and downstream of the EGR valve, and the bypass passage is coupled to an engine exhaust downstream of the exhaust throttle. 
     
     
       8. The method of  claim 1 , wherein the bypass passage includes an ejector and wherein routing the portion of throttled exhaust gas via the bypass passage includes flowing the portion of throttled exhaust gas through the ejector, the method further comprising drawing vacuum at the ejector. 
     
     
       9. The method of  claim 4 , further comprising, while the temperature of the exhaust catalyst is below the threshold temperature and while the exhaust throttle is closed, retarding spark ignition timing, an amount of spark retard adjusted based on the temperature of the exhaust catalyst. 
     
     
       10. The method of  claim 9 , further comprising, after the temperature of the exhaust catalyst is above the threshold temperature, maintaining the exhaust throttle closed while advancing spark ignition timing. 
     
     
       11. The method of  claim 10 , further comprising, after the temperature of the exhaust catalyst is above the threshold temperature, adjusting the exhaust throttle based on an EGR cooler outlet temperature. 
     
     
       12. The method of  claim 11 , wherein the adjusting is during a cold-start and includes, as the outlet temperature of the EGR cooler increases, shifting the exhaust throttle from a more closed position to a more open position. 
     
     
       13. The method of  claim 1 , wherein the EGR valve is included in the EGR passage coupling an engine exhaust, upstream of the exhaust throttle, to an engine intake, and wherein the EGR passage includes the EGR cooler coupled downstream of a juncture of the EGR passage and the engine exhaust and upstream of the EGR valve. 
     
     
       14. The method of  claim 13 , wherein an exhaust ejector is located in the bypass passage coupling the EGR passage, downstream of the EGR cooler, to the engine exhaust, downstream of the exhaust throttle. 
     
     
       15. The method of  claim 14 , wherein the coordinated closing of the exhaust throttle increases vacuum generation at the exhaust ejector and flowing a portion of an amount of exhaust gas through the EGR cooler and then through the exhaust ejector, and drawing vacuum generated by the flowing exhaust at the ejector. 
     
     
       16. The method of  claim 15 , wherein the exhaust ejector is coupled to a vacuum actuator, and wherein the closing of the exhaust throttle is performed in response to actuation of the vacuum actuator. 
     
     
       17. A method for an engine, comprising:
 during an engine cold-start, closing a post-catalyst exhaust throttle and an EGR valve while diverting at least a portion of throttled exhaust gas through an EGR cooler, the EGR cooler coupled to an exhaust passage upstream of the exhaust throttle, the EGR valve coupled to an EGR passage downstream of a junction, the diverted portion directed to a bypass passage; and 
 during engine operation, coordinating closing of the exhaust throttle and the EGR valve to maintain an amount of EGR delivered to the engine, wherein the coordinated closing of the EGR valve includes fully closing the EGR valve. 
 
     
     
       18. A method for an engine, comprising:
 during an engine cold-start, closing a post-catalyst exhaust throttle and an EGR valve while diverting at least a portion of throttled exhaust gas through an EGR cooler, the EGR cooler coupled to an exhaust passage upstream of the post-catalyst exhaust throttle, the EGR valve coupled to an EGR passage downstream of a junction, the diverted portion directed to a bypass passage; and 
 after the cold-start, during engine operation, operating the engine with exhaust gas recirculation, and in response to a vacuum requirement,
 reducing an opening of each of the post-catalyst exhaust throttle and the EGR valve to increase exhaust flow through the EGR cooler; and 
 drawing vacuum at an ejector.